# Question Video: Understanding Newton’s First Law of Motion Physics • 9th Grade

An object travels between point A and point B, where the points are separated by a distance of 2 meters. The object travels this distance in a time interval of 3 seconds and is moving at a constant velocity throughout the time interval. Which of the following must be false? [A] No forces act on the object. [B] The magnitude of the net force that acts on the object does not change during the time interval. [C] The speed of the object is constant. [D] The speed of the object is zero.

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### Video Transcript

An object travels between point A and point B, where the points are separated by a distance of two meters. The object travels this distance in a time interval of three seconds and is moving at a constant velocity throughout the time interval. Which of the following must be false? (A) No forces act on the object. (B) The magnitude of the net force that acts on the object does not change during the time interval. (C) The speed of the object is constant. (D) The speed of the object is zero.

Okay so in this question we have an object that travels between two points that we’re told are separated by a distance of two meters. We’re told that the object takes a time of three seconds to travel this distance and that it moves at a constant velocity throughout the time interval. We are then given four different possible statements about the object, and we’re asked to work out which of them must be false.

In order to answer this question, we’re going to find it helpful to recall a law known as Newton’s first Law of motion. This law says that an object at rest remains at rest and an object with a constant velocity continues to travel at that velocity unless it’s acted on by an unbalanced force. Newton’s first law means that if the forces on an object are balanced or there’s no net force on the object, then that object moves with a constant velocity. If the object isn’t initially moving, then its velocity remains constant at zero. If it is initially moving, then its velocity remains constant at a nonzero value. This statement also applies the other way around.

So if an object is moving with a constant velocity, whether or not that velocity is zero, then there must be zero net force acting on the object. Since we’re told in the question that the object moves at a constant velocity throughout the time interval, this means we know that throughout this interval there is no net force acting on the object. With all this in mind, let’s take a look at each of the four possible statements that we’ve been given.

The first statement in option (A) is that no forces act on the object. Now, we’ve seen that there must be no net force acting on the object. And it turns out that there’s two possible ways this could happen. No net force could either mean that no forces act on the object at all. Or it could mean that multiple forces act on the object but that those forces are balanced so that they cancel each other out. This means that the statement given in option (A) that no forces act on the object may or may not be true. So we’ve labeled this statement as could be true.

Next, let’s look at the statement given in option (B). This says that the magnitude of the net force that acts on the object does not change during the time interval. We know that this object has a constant velocity throughout the whole of this interval and that a constant velocity means no net force. So the net force on the object must be zero for the whole of the interval. In other words, that net force remains constant or doesn’t change during the time interval. So the statement in option (B) that the magnitude of the net force on the object does not change during the time interval must be a true statement.

Now, let’s look at the statement given in option (C). This says that the speed of the object is constant. Now, we know from the question that the object has a constant velocity. And if the object’s velocity is constant, then its speed must also be constant. So the statement given in option (C) must be true.

This leaves us with the statement in option (D) that the speed of the object is zero. We’re told that the object travels a distance of two meters in a time of three seconds. And we can recall that the speed of an object is defined as the distance moved by that object divided by the time taken to move that distance. In this case, since the distance moved is not zero, then the speed of the object cannot possibly be zero. And so the statement in option (D) that the speed of the object is zero must be false.

This means that our answer to the question of which of the following must be false is that it’s the statement given in option (D) that the speed of the object is zero.